Recently, since highly precise and compact digital cameras, camera-equipped mobile-phones, and the like have been popularized, demands for weight saving and miniaturization of optical systems have been rapidly increased. In order to meet these demands, an optical design using a highly functional glass aspheric lens becomes the mainstream. In particular, a large-aperture aspheric lens using a glass showing a high refractive index and a low dispersion characteristic is important for the optical design.
The highly functional glass aspheric lens is produced by a molding method which forms an optical surface directly by press molding and does not require grinding and polishing steps. In the molding method, a glass molding preform is pressed under a high temperature. Therefore, the preform and the mold are exposed to a high-temperature and high-pressure environment. Accordingly, it is desired to suppress the glass transition temperature (Tg) and yield point temperature (Ts) of the preform to low values.
As a glass showing a high refractive index and a low dispersion characteristic, a glass containing B2O3 and La2O3 as main components is known. However, there have been a problem that the life of a noble metal-based protective film formed on a tungsten carbide (WC)-based mold base metal is short and the durability of the mold for molding does not last long since the molding temperature is generally high, and a problem that a molding cycle is long and thus productivity is low.
In order to solve the above problems, a glass containing, as main components, Li2O in addition to B2O3 and La2O3 is known. However, since it contains a large amount of rare metal elements such as La2O3, there is a problem that a stable glass is not obtained when it is intended to increase the refractive index.
In order to solve the above problems, a glass containing B2O3—SiO2—La2O3—Gd2O3—ZnO—Li2O—ZrO2 as main components has been proposed in Patent Document 1. However, a composition of a high-refractive-index glass having a refractive index of 1.79 or more is not specifically disclosed in Examples. It also involves a problem that the molding temperature is high.
Further, Patent Documents 2, 3, and 4 propose optical glasses for mold press molding containing B2O3—SiO2—La2O3—ZnO—Li2O—ZrO2—Ta2O5 as main components and having nd of 1.84 or more, an Abbe number vd of 35 or more, Tg of 630° C. or lower. However, the glasses excessively contain Ta2O5 and Gd2O3, so that they involve problems when trying to achieve low specific gravity, high refractive index, and cost saving.    Patent Document 1: JP-2003-201143    Patent Document 2: JP-2003-267748    Patent Document 3: JP-2006-016293    Patent Document 4: JP-2006-016295